Monday, December 22, 2008

Zooming in on an early galaxy

One of the key questions about galaxies concerns the process in which they form. Galaxies are made up of stars, but in general stars do not form in isolation – except for the very first stars in the universe, which we discussed here.

More normally, stars form simultaneously in close proximity to each other as part of the larger process of galaxy formation. But galaxy formation can happen in one of at least two different ways. In the first case, the baryonic matter that will eventually form stars gradually contracts under gravitational force along with the larger mass of dark matter in which it is "trapped". This is a gradual process in which the entire galaxy is formed as the baryonic matter eventually collapses to a state that is dense enough for individual stars to form, much as stars form even today, albeit quite slowly, as in the Milky Way of the present time. (The present rate is only 2 or 3 per year on average.)

Alternatively, in the second case, relatively small groups of stars form within irregular protogalaxies, which then over long periods of time (billions of years) merge with each other to form galaxies as we are familiar with them today.

The first scenario is typically called "monolithic collapse" or formation by "gas accretion". The second is called the "hierarchical" or "merger" scenario.

Of course, both processes can occur. Conceivably some of the large galaxies we can see today were built up from the merger of many smaller galaxies, while others formed more or less in isolation. But the interesting question is whether one or the other of these scenarios was more common in the early days of the universe. (Since at the present time most matter available to feed galaxy growth is already part of a galaxy, most galaxy growth now is by merger.)

In the very early universe, that is for the first 2 or 3 billion years after the big bang (which was about 13.7 billion years ago), there would have been less opportunity for galaxy mergers, so one might expect that more of the galaxies back then formed in isolation rather than in a hierarchical process of mergers.

As explained in this article on redshift, galaxies that formed within the first 3 billion years after the big bang all have redshifts of about 2.2 or more.

Unfortunately, at such redshifts, the objects are at distances of 10.7 billion to more than 13 billion light-years, so it's nearly impossible to make out any details, even with the most powerful existing telescopes, and even using sophisticated technology like adaptive optics. Therefore, there's very little reliable information about the actual morphology of early galaxies, making it impossible at present to obtain enough evidence to decide between the two scenarios.

In particular, it's certainly not possible to discern the overall shape of such a distant galaxy, to determine whether it's an ordinary spiral, a more shapeless elliptical galaxy, or something else, such as a pair of merging galaxies. Morphological information is what one needs in order to discriminate between nice orderly spiral galaxies and galaxies that are distorted due to mergers.

Even though situating a telescope in space, such as the HST, or on the Moon solves the problem of atmospheric distortions, the only way to get better resolution is by using a larger mirror. Telescopes with larger optics are being planned, but fortunately nature itself provides some help even today, albeit in very rare circumstances, as we'll explain in a moment.

Although we can't easily determine the properties of such early galaxies, at least it's possible to find them relatively easily in surveys, without even having to study them spectroscopically. We just discussed the technique for finding these early galaxies by the Lyman break techniques.

It turns out that even though we cannot see the structure of these Lyman-break galaxies directly, we can do spectroscopic measurements that tell us a surprising amount of information about these galaxies. This information can tell us about galaxy rotation, whether there are strong outflows of gas (due to stellar winds or supernova ejecta), and even the approximate rate of star formation in the galaxy.

One thing that spectroscopic data allows astronomers to do is to map "velocity fields" in different parts of a galaxy. That is, we can determine how fast specific parts of the galaxy are moving relative to the galaxy as a whole. For example, in a rotating spiral galaxy, we should detect that the half of the galaxy rotating away from us has an additional redshift beyond the overall redshift of the galaxy, while the other half has a slightly lower redshift. If this pattern varies in a regular way from one end of the galaxy to the other, we can be fairly sure the galaxy is a spiral. On the other hand, if the relative velocities of different parts of the galaxy are irregular or "chaotic", we probably have a galaxy without much regular structure.

However, even this velocity field information we have is at a rather low resolution compared to the size of the galaxy. For historical reasons, astronomers customarily use a length scale called a parsec, or "parallax arcsecond", which is about 3.26 light-years. The resolution, for spectroscopic purposes, of an object at redshift z≳2.2 is about 1300 pc (parsecs), or 4000 light-years, and this is more than half the size of the visible part of a typical Lyman-break galaxy. (Our Milky Way is a lot larger, roughly 100,000 light-years in diameter.)

What can be done about this? Not a whole lot, actually, until we have telescopes with much better resolution – except in certain very special circumstances. Those circumstances exist when another large galaxy or galaxy cluster lies directly in the line of sight between us and the very distant galaxy we're interested in. Then we have what is called a gravitational lens. This works pretty much like an ordinary optical lens, because according to general relativity massive objects are capable of bending light.

A group of astronomers have now examined one example of exactly this circumstance. The distant galaxy in question is named J2135-0102 (a catalog number). It lies at z=3.075. So we see it as it was 2.13 billion years after the big bang. We know that there were already galaxies much less than a billion years after the big bang, so this one isn't that unusual.

Because of this gravitational lens, it has been possible to map the velocity field of J2135-0102 with a resolution of ~120 pc, about 5 times better than possible without the gravitational lens. The abstract of the research paper reveals what can be learned from this:

The formation and assembly of a typical star-forming galaxy at redshift z ≈ 3
Recent studies of galaxies ~2ā€“3 Gyr after the Big Bang have revealed large, rotating disks, similar to those of galaxies today. The existence of well-ordered rotation in galaxies during this peak epoch of cosmic star formation indicates that gas accretion is likely to be the dominant mode by which galaxies grow, because major mergers of galaxies would completely disrupt the observed velocity fields. But poor spatial resolution and sensitivity have hampered this interpretation; such studies have been limited to the largest and most luminous galaxies, which may have fundamentally different modes of assembly from those of more typical galaxies (which are thought to grow into the spheroidal components at the centres of galaxies similar to the Milky Way). Here we report observations of a typical star-forming galaxy at z = 3.07, with a linear resolution of ~100 parsecs. We find a well-ordered compact source in which molecular gas is being converted efficiently into stars, likely to be assembling a spheroidal bulge similar to those seen in spiral galaxies at the present day. The presence of undisrupted rotation may indicate that galaxies such as the Milky Way gain much of their mass by accretion rather than major mergers.

So, the researchers have concluded that in the case of this small, but typical, Lyman-break galaxy, a fairly regular structure is present, and it is more likely due to gas accretion rather than hierarchical assembly.

That's the big news, but the researchers were able to deduce a few other things as well.

For example, from the rotation curve, J2135-0102 appears to have a mass of ~2×109 M (solar masses) within a radius of 1800 pc. (The Milky Way, by comparison, is about 5.8×1011 M.) The galaxy's rate of star formation has also been estimated at 40±5 M per year – much higher (per unit volume) than the Milky Way's rate of only 2 or 3 M per year. This high rate, however, does seem to be typical of the rate in other Lyman-break galaxies that have been studied.

Further reading:

Cosmic Eye Sheds Light On Early Galaxy Formation, Just Two Billion Years After Big Bang (10/8/08) – press release

Cosmic eye telescope used to spot distant galaxy (10/8/08) - news article in the Telegraph (UK)



ResearchBlogging.org
Daniel P. Stark, A. Mark Swinbank, Richard S. Ellis, Simon Dye, Ian R. Smail, Johan Richard (2008). The formation and assembly of a typical star-forming galaxy at redshift zā€‰ā‰ˆā€‰3 Nature, 455 (7214), 775-777 DOI: 10.1038/nature07294


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2 Comments:

Anonymous KAS said...

Nice post, thanks! I'll be linking it.

Happy Holidays!

KAS

12/24/2008 07:07:00 AM  
Blogger Charles Daney said...

Thanks for the kind words and the link. Glad you liked the article.

12/24/2008 12:08:00 PM  

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